1. Field of the Invention
The present invention relates to terminal equipment for telecommunications and information processing with improved electromagnetic compatibility (EMC), and more specifically, to terminal equipment for telecommunications and information processing which has a human-machine interface (HMI) and a metallic telecommunications interface (TCI).
2. Description of the Related Art
Electromagnetic compatibility of terminal equipment for telecommunications and information processing, including telecommunication terminal equipment (TTE) and information technology equipment (ITE), has been strongly demanded these days, from practical viewpoints as well as in some legal aspects.
Publication No.22 issued by the International Special Committee on Radio Interference (CISPR) in the International Electrotechnical Commission (IEC) provides standards specifically concerning electromagnetic interference (EMI) for ITE. Another publication CISPR No.24, a draft standard under review, describes about immunity from electromagnetic interference. EMC-related legislation in many countries is based on those international standards, and the TTE and ITE are now about to become subject to the regulation requiring conformity to their relevant standards.
As described above, the requirement of electromagnetic compatibility in the terminal equipment for telecommunications and information processing (hereafter, shortened as "info-telecom equipment") has two aspects: immunity and interference. First, the immunity is defined as a tolerance to attack by electromagnetic waves which may be available in a predetermined electromagnetic environment, where the info-telecom equipment must operate as designed without any problems. Second, the interference calls for limitation of electromagnetic disturbance to the environment at a predetermined level.
For more practical example, the info-telecom equipment is required to exhibit enough immunity to an approaching citizens band (CB) radio device with high power outputs violating the regulations. It is also demanded to have a prescribed immunity to an industrial-scientific-medical (ISM) device installed nearby. Note that those noise-generating devices would produce a field strength as high as 1 V/m to 10 V/m. In addition to such immunity performance, the info-telecom equipment is regulated not to produce too much electromagnetic noises that could cause audible noises, beat disturbance, or malfunctions of television sets and/or radio receivers in the neighborhood.
The info-telecom equipment subject to the EMC regulation includes a vast range of devices encompassing telecommunications terminal equipment such as analog and digital telephone sets and facsimiles, and information technology equipment such as personal computers, word processors, and computer's dedicated I/O terminals.
Take analog/digital telephone sets for instance. Their main bodies are connected to human-machine interface (HMI) devices such as handsets and headsets, as well as being wired to a public telephone network through a metallic telecommunications interface (TCI). Likewise, the information technology equipment is often coupled to some HMI devices such as keyboards, display units, microphones, and speakers, besides being linked to TCI facilities through modems for an analog circuit or through terminal adapters for a digital circuit. As such, the info-telecom equipment in general has connections to both HMI and TCI, and in many cases, has a power line interface (PLI), through which the equipment is supplied with its main power from a commercial power source.
Those HMI and TCI connection lines, however, may work as antennas to catch some electromagnetic noises or to emit them to the outside space. Therefore, when thinking about measures to be taken for solving the EMC problems (including both immunity and interference) in the info-telecom equipment, it must cover not only the main body of the equipment but the entire system including the HMI and TCI connections.
FIG. 31 is a diagram showing an outline of the info-telecom equipment to describe EMC problems concerned therewith. As previously explained, the main body of telecommunications terminal 110 is connected to an HMI unit 120 via HMI cables 131 and 132. The terminal 110 is also connected to a public or private communication network 160 via a TCI cable 141 and to an AC power supply 150 via a PLI cord 142.
The telecommunications terminal 110 consists of the following circuits: a receiver 111 and a driver 112 for interfacing with the HMI unit 120; an internal circuit 113 for main processing; a driver 114, a receiver 115, and a hybrid circuit 116 for interfacing with the TCI facilities. The internal circuit 113 is organized by a signal processing circuit, clock oscillators, and a CPU chip. This kind of circuit may malfunction if attacked by some external noise signals that exceed an allowable level or contain some frequency components which is particularly critical in their mutual interference. In turn, the components in the internal circuit 113 can be a source of electromagnetic noises that may affect operations of peripheral devices. Returning to FIG. 31, the HMI unit 120 contains a transmitter 121 and a receiver 122.
When an electromagnetic noise source 161 is closely located to the HMI cable 131, some electromagnetic noise signals will enter the telecommunications terminal 110 as indicated by a broken line A1. Similarly, when further electromagnetic noise sources 162 and 163 approach the TCI cable 141 and the PLI cord 142, some noise signals may go into the terminal 110 as indicated by broken lines A2 and A3, respectively.
On the other hand, a noise current having leaked from the internal circuit 113 to the HMI cable 132 will cause an emission of electromagnetic noise toward the outside of the terminal 110 as indicated by a broken line B1. Likewise, another noise current from the internal circuit 113 to the TCI cable 141 would result in an electromagnetic noise emission as indicated by a broken line B2.
The following description will present the detailed propagation paths of electromagnetic noise signals. FIG. 32 is a diagram showing such possible paths along which the noise goes into the telecommunications terminal 110. In addition to the aforementioned receiver 111, internal circuit 113, and driver 114, FIG. 32 shows two more components in the terminal 110: a common-mode choke coil Lc1 inserted between the HMI cable 131 and receiver 111, and another common-mode choke coil Lc2 inserted between the TCI cable 141 and driver 114.
An input signal source Eh is connected at the far end of the HMI cable 131, and another input signal source Et is provided at the far end of the TCI cable 141. There are two more signal sources in FIG. 32, which voltages are developed over the earth potential, being induced by external electromagnetic noise sources. Under the assumption that the two induced voltages have nearly the same amplitude, a symbol Un is assigned to both of them. The signal ground SG and frame ground FG of the telecommunications terminal 110 are connected to each other at a single point and those two ground potentials have a capacitive coupling to the ground through a stray capacitance Cs.
The route of noise currents that flow into the terminal will be now discussed below. The induced voltage Un at the HMI cable 131 can be 1 V to 3 V in some cases that a portable telephone or CB radio device is placed near the terminal. The Un noise current enters the internal circuit 113 through the HMI cable 131 and internal coupling capacitance Cc1 and Cc2 (as indicated by broken lines) and flows away to the ground via the signal ground SG, frame ground FG, and stray capacitance Cs.
Another induced voltage that appears on the TCI cable 141 has a similar voltage level and it causes a noise current that circulates in a loop formed by coupling capacitance Cc3 and Cc4, signal ground SG, frame ground FG, and stray capacitance Cs.
Generally speaking, because of a difference in capacitance between Cc1 and Cc2 and some other reasons, the impedance of the two signal conductors of the HMI cable 131 are not balanced with respect to the ground, thus causing a difference in their respective currents from the common noise source Un (i.e., a difference in common-mode currents). This unbalanced circuit scheme produces a difference in the noise voltages observed at two input terminals of the receiver 111 with respect to the ground potential, and it will cause a variation in its envelope detection signal because of non-linearity of input characteristics of the receiver 111. That is, the induced noise voltage Un results in a demodulation noise voltage (i.e., a differential voltage between the two signal conductors), which will degrade the S/N ratio in reception of the input signal Eh. This can be a cause of interference troubles in the case of analog HMI or deterioration of bit error rates (BER) in the case of digital HMI. The same problems as described above can happen to the driver 114 at the TCI side of the terminal.
Further, when the frequency of the electromagnetic noise source is equal to the internal clock frequency or bus cycle frequency or expressed as an integral multiple (or fraction) thereof, a beat may occur and it sometimes leads to a malfunction of counters and dividers contained in the internal circuit 113.
Use of common-mode choke coils is one of the traditional techniques that have been widely adapted as a countermeasure to the above noise problems. In the telecommunications terminal of FIG. 34, for example, the common-mode choke coils Lc1 and Lc2 are placed at the connection points of the HMI cable 131 and TCI cable 141. However, those choke coils do not provide enough performance for the reasons described below.
FIG. 33 is a diagram schematically representing the telecommunications terminal 110 and associated cables in FIG. 32 as an equivalent antenna circuit. FIG. 33 shows how the terminal 110 with the cables 131 and 141 will behave when they are exposed in a strong noise field. According to this figure, one end of the HMI cable 131, at least, is electrically open with respect to the ground, while the body of the terminal 110 has a certain impedance to the ground lower than that of the HMI cable 131 due to its stray capacitance Cs. As a result, a standing wave Ua is produced on the HMI cable 131, which wave has a voltage loop at the open end of the HMI cable 131 and a voltage node at the body of the terminal 110.
On the other hand, the noise wave induced on the TCI cable 141, which is longer than the HMI cable 131 in most cases, propagates through the body of the terminal 110 and reflects at the open end of the HMI cable 131. This also causes a standing wave Ub, continuous to the aforementioned standing wave Ua, whose voltage loop is at the point one half-wavelength away from the end of the HMI cable 131. The voltage node of the standing wave Ub is located at the body of the terminal 110. As such, the system shown in FIG. 32 can be expressed in the equivalent antenna system in FIG. 33.
When the electrical length (i.e., the length of a conductor in terms of wavelength) of the HMI cable 131 including a loading effect of the common-mode choke coils Lc1 and Lc2 is an odd multiple of quarter-wavelength, the antenna system shown in FIG. 33 will resonate at that frequency, thus causing an induced noise current flowing through the telecommunications terminal 110 as indicated by the broken line I. This noise current will then produce maximum amplitude of voltage drop Vi at the internal circuit 113.
More specific calculations will be now presented, taking the following three examples for the electrical length of the HMI cable 131 including common-mode choke coils in the terminal 110. When the total electrical length is about 2.8 m, 1.5 m, or 0.5 m, the noise immunity of the terminal 110 will be expected to deteriorate at the frequencies 27 MHz (CB radio service), 50 MHz (amateur band), or 144 MHz (amateur band), respectively. It is because the above-listed electrical lengths coincide with one quarter-wavelength at those frequencies. Likewise, the noise immunity may be degraded at 80 MHz (FM radio), 150 MHz (police radio), or 430 MHz (amateur band), at which frequencies the above-listed electrical lengths correspond to three quarter-wavelengths.
In general, antenna systems will reversibly work for both transmission and reception of radio signals. Thus the HMI and TCI cables 131 and 141 also serve as a transmission antenna that will radiate spurious radio waves to the surrounding air if there is any internal noise source. As a matter of fact, a broadband noise source does exist inside the telecommunications terminal 110, such as clock oscillators and their divided frequency signals. FIG. 34 schematically shows such behavior as a transmission antenna of the telecommunications terminal 110 in FIG. 32. A noise source Vn in FIG. 34 represents clock oscillators and the like which may cause spurious radiation.
Similarly to the mechanism of external noise reception, the spurious radio waves are emitted from the equivalent antenna system of FIG. 34. When the electrical length of the HMI cable 131 including a loading effect of the common-mode choke coils Lc1 and Lc2 equals any odd multiple of quarter-wavelength of the internal noise signal, the antenna system resonates and thus radiates spurious waves at enhanced strength.
As a result of resonance, such spurious emission can sometimes exceed a tolerance level defined in some EMI regulations. The above discussion has clarified some EMC problems with telecommunications terminal equipment, however, the same problems can happen to information technology equipment having communications capabilities integrated therein or externally added thereto. A PLI cord, when attached, will function as part of the antenna element in the same way as the TCI cable.
As described above, since the HMI and TCI cables or PLI cord may serve as a reception antenna, the info-telecom equipment is likely to be affected by an external source of electromagnetic noise. Also, since the same cables act in turn as a transmission antenna resonating at internal noise frequencies, it makes an enhanced radiation of electromagnetic noise to outside. Those EMC problems, however, have not been solved yet by conventional techniques.